Recent technological developments in direct access storage devices, and in particular in magnetic storage devices have dramatically increased storage density. As a result, new storage devices having disks of smaller diameter can hold much more information.
As is well known in the art, the magnetic disk containing the information has circular tracks. One or more disks form a stack which is rotated by a motor. A read/write head is disposed in close facing relationship to a surface of the disk, so that digital information may be stored on or read from the tracks. Generally the surface of the disk is divided into sectors. Each track has, for each sector, a region which is used to store information which identifies the particular track and sector. The detection of this information by the read/write head and subsequent processing by read/write electronics and a decoder produces position error signals which are interlaced in a stream of data read from the disk. This is the case for a disk file having sector servo architecture. In a dedicated servo, all position information is stored on a single disk surface, while all other disk surfaces have only data information. Sometimes, the data surfaces may have a few tracks of position information, usually at the edges of the annular data band, to handle offsets between the servo head and any of the data heads.
The position error signal (PES) is used in a servo control loop to control the position of an actuator, on which the read/write head is mounted. With suitable inputs to the servo control system, the position actuator be directed to a particular track and, once having found that track, will be directed to follow the track.
As overall disk size is decreased, the position error signal occupies an increasing percentage of the total area of the disk, thus decreasing the effective data storage density. Further, as disk size decreases, it is theoretically possible to obtain faster access times to a desired track and sector. However, if the number of regions containing position error signal information is decreased to maintain the highest possible storage density, the advantage of faster access times is lost. The loss of faster access times is a direct result of the fact that prior art systems must wait until new position error signal information is obtained, in order to recompute the amount of energy that must be supplied to the actuator that positions the read/write head. Since the frequency of occurrence of the position error signal information is not high, and computation of the amount of energy required to meet a particular velocity profile in seeking a track is performed only at the receipt of a new position error signal, prior art systems are not as fast and accurate, in seeking and following a particular track, as would seem to be possible in theory. Settle-out times (that is, the time for the head to settle within a certain error band of a target track) are limited by waiting for position error signal information, rather than by mechanical constraints, thus limiting the speed with which data can be accessed.